Programmed cell death or apoptosis is a universal mechanism that operates in multicellular organisms to regulate cell number and tissue homeostasis. Whereas the effector phases of apoptosis appear to be conserved from worms to mammals, the molecular nature of endogenous signaling factors that instruct cells to undergo apoptosis is largely unknown. Our preliminary data indicate that such instructions may be sent by the Frizzled receptors that are activated by the Writ family of secreted proteins. Thus, in addition to previously characterized role of Writ and Frizzled in the control of cell proliferation and cell fate determination, this signaling pathway may function to trigger apoptosis during development or in disease. Experiments proposed in this application will characterize the novel mechanisms of the cell death program triggered by the Frizzled receptors. These studies will be carried out in the Xenopus model system that allows a combination of the molecular genetic approach with rapid functional assays in vivo. The gain-of-function and the loss-of-function approaches will be utilized to define molecules, which are necessary and sufficient for signal transduction during Frizzled-dependent apoptosis. Mutagenesis of Frizzled receptors combined with the structure-function analysis of their ability to induce apoptosis will be carried out. Proteins that physically and functionally interact with Frizzled domains essential for apoptotic signaling will be identified. Finally, a role for endogenous Wnt/Frizzled signaling in apoptosis will be assessed in vivo using specific pathway inhibitors. Proposed studies will identify the components involved in this novel pathway and should decipher new ways of controlling cell number in development and in disease.